<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <h1 id="app">hello</h1>
    <script>
        var app =  document.getElementById("app");
        console.log(app);
        // 获取元素内容
        console.log(app.innerHTML);
        // 修改内容
        app.innerHTML = "FUCK";
    </script>
</body>
</html>